Alloy for fuel of neutronic reactors



United States Patent ALLOY FOR FUEL 0F NEUTRONIC REACTORS Clarence H. Bloomster and Yeichi B. Katayama, Richland, Wash, assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Sept. 21, 1960, Ser. No. 57,594

Claims. (Cl. 204154.2)

This invention deals with a plutonium-containing aluminum-base alloy of high corrosion resistance, with the process of making the alloy and with a fuel element for neutronic reactors having a core of the alloy.

In many neutronic reactors, for instance in the Plutonium Recycle Test Reactor, the fuel has to be replenished from time to time after the reactor has operated for a while and some of the fuel has been consumed. Also sometimes it is desirable to spike uranium fuel by the addition of plutonium-containing material. In order to replenish or enrich the fuel, so-called fuel spike elements are inserted into the reactor zone proper. For these fuel spike elements, cores of plutonium-aluminum alloys have been used that were clad in a corrosion-resistant zirconium alloy.

As will be readily understood, it is of prime importance that the cladding of such fuel spike elements has a high degree of corrosion resistance, because it is in contact with the cooling *water in the reactor. However, it is also necessary that the fuel proper or the core is corrosion-resistant, because sometimes leaks or minor cracks occur in the cladding and then the cooling water can penetrate through the cladding to the fuel proper and react with the latter. This would create an extremely troublesome situation. In the Plutonium Recycle Test Reactor the cooling water averages a temperature of about 350 C. and it thus is cycled at superatmospheric pressure which intensifies the reaction of the fuel in case of a leak. The Plutonium Recycle Test Reactor is described in detail in report HW-62000 published by General Electric Company.

It is an object of this invention to provide alloys that are suitable as fuel material for neutronic reactors and that are highly corrosion-resistant when in contact with comparatively hot water under superatmospheric pressure.

It is another object of this invention to provide alloys suitable as fuel for neutronic reactors that have a comparatively low neutron-capture cross section.

It is still another object of this invention to provide plutonium-containing alloys that can be cast While in contact with air.

It is also an object of this invention to provide a plutonium-containing aluminum alloy that can be fabricated easily, for instance, by extrusion.

It is still another object of this invention to provide a plutonium-containing aluminum alloy that is stable to radiation prevailing in neutronic reactors.

It is still a further object of this invention to provide plutonium-containing aluminum alloys that have a good thermal conductivity.

It is finally an object of this invention to provide plutonium-containing aluminum alloys that withstand repcated temperature changes of between 100 and about 400 C., so-called thermocycles, as they occur in neutronic reactors.

It has been found that, if nickel is added to the binary plutonium-containing aluminum alloy, the corrosion resistance is greatly improved. A percentage of from 2 to 3.5% nickel yields the highest corrosion resistance, and within this range a content of 2.0% was preferred. (All percentages in this specification are expressed as percent by weight.)

3,086,930 Patented Apr. 23, 1963 It has been furthermore found that, if part of the nickel is replaced by silicon, the corrosion resistance remains at least the same as when the higher nickel content specified above is added; but this alloy has the additional advantage of a lower neutron-capture cross section, since silicon has a considerably lower cross section than nickel. On the other hand, the ternary aluminum-plutonium-nickel alloy can be processed more readily by the aqueous methods that are conventionally used for the processing of neutron-bombarded fuel elements.

More specifically, the invention deals with ternary aluminum-base alloys that contain plutonium in a quantity of from 1 to 10% and nickel in a quantity of from 2 to 3.5%, and it deals with a quaternary aluminum alloy cont aining from 1 to 10% of plutonium, from 1.0 to 1.4% of nickel and from 0.9 to 1.1% of silicon. In addition, both types of alloys of this invention may contain small quantities, up to 0.7%, of iron. The very best results were obtained with an alloy containing 1.8% of plutonium, 1.2% of nickel, 1.0% of silicon and less than 0.7% of iron.

It was also found that, in order to obtain optimum corrosion resistance, the particles of the second phase should be as small as possible and should be uniformly dispersed in the aluminum. This end is accomplished by melting the alloy and chill-casting it from a temperature of between 725 and 750 C. into a cold (about 25 C.) graphite mold so that solidification occurs rapidly; under these conditions, casting can be carried out in air.

In the following, a few examples of the alloys of this invention are given for illustrative purposes. For these examples, 2-inch long samples were cut from 0.5-inch thick extruded rods.

Example I A sample consisting of aluminum and 1.8% of plutonium was heated in an autoclave at 350 C. and an initial pressure of 2400 psi. in neutral, deionized water. At the end of 24 hours the sample had completely corroded to a powdery mass. (Due to evolution of hydrogen, the pressure increased considerably during the treatment.)

Example II Another sample of a quaternary aluminum alloy containing 1.8% plutonium, 1.3% nickel and 1.1% silicon was heated in the autoclave for a total of 96 hours at 350 C. and 2400 p.s.i. in neutral, deionized water. At the end of the 96-hour period, the sample was found to be corroded to a uniform depth of 0.29 milli-inch.

Example III In order to approximate the conditions in the Plutonium Recycle Test Reactor, a sample of the same alloy as that employed in Example II was treated under the same conditions of temperature and pressure, but using deionized water adjusted to a pH of 10 by the addition of lithium hydroxide. After 96 hours the sample was found to be corroded to a uniform depth of 0.19 milli-inch.

Example IV A sample of a ternary alloy of aluminum, 1.8% plutonium and 2.0% nickel was treated as described in Example II. Visual inspection after 96 hours showed only the typical aluminum oxide film.

Example V Another sample of the alloy described in Example IV was heated in deionized water adjusted to a pH of 10 with lithium hydroxide, using the same temperatures and pressures as in Example II. After 72 hours, it had corroded to a uniform depth of 0.6 milli-inch.

As is mentioned, the alloys of this invention are primarily intended for fuel spike elements for neutronic reactors. One method preferred by the inventor for the preparation of the fuel elements consists of heating the alloy to about 725 C. and casting the melted alloy into a graphite mold of room temperature. The mold that was used had an inner diameter of 2.5 inches and a height of 11 inches. The billets formed thereby showed such good surface qualities that machining was not necessary, and the structure showed little porosity and could be extruded easily.

The billets were extruded to a diameter of 0.5 inch using flat-face shear dies, a reduction ratio of 27:1, a temperature of about 500 C. for both, billet and container, and a ram speed of 20 inches per minute. A suspension of colloidal graphite in oil was used as the lubricant. The average peak extrusion pressure was 80,500 psi. The extruded rods were about 200 inches long. They were cut in halves, straightened in a roll-rod straightener and cut to the desired fuel length of 88 inches by a rotary cut-off saw. The rods were then gauged for their diameter and their length.

Tubing made of corrosion-resistant zirconium alloy, for instance an alloy containing 1.5% of tin, 0.15% of iron, 0.10% of chromium, nickel up to 0.07% and the balance consisting of zirconium, was used for the cladding of the fuel core; the wall thickness of the cladding was 0.030 inch. A cap was welded to one end of the cladding tube in a welding chamber that was filled with an inert gas, such as nitrogen. The tubes fitted loosely around the fuel rods, and the space between the rods, and the cladding was filled with helium. Then a second cap was mounted to the upper end and hermetically sealed thereto. A number of the fuel elements just described were assembled and inserted as a spike assembly into a Plutonium Recycle Test Reactor.

It will be understood that this invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claims.

What is claimed is:

1. A corrosion-resistant ternary alloy consisting of from 1 to by Weight of plutonium, nickel, and from 2 to- 3.5% by weight of aluminum.

2. The alloy of claim 1 in which the plutonium content is about 1.8% by weight and the nickel content about 2.0% by weight.

3. A corrosion-resistant quaternary aluminum alloy consisting of from 1 to 10% by weight of plutonium, from 1 to 1.4% by weight of nickel, from 0.9 to 1.1% by weight of silicon and aluminum.

4. The alloy of claim 3 in which the plutonium content is 1.8% by weight, the nickel content is about 1.2% by Weight and the silicon content about 1.0% by weight.

5. A fuel element for neutronic reactors having a high corrosion resistance to water of elevated temperature and superatmospheric pressure, consisting of a core and a jacket around said core, said core consisting of an alloy composed of from 2 to 3.5% by weight of nickel, from 1 to 10% by weight of plutonium, and aluminum and said jacket consisting of a corrosion-resistant zirconium alloy.

6. The fuel element of claim 5 wherein the core consists of 1.8% by weight of plutonium, 2% by weight of nickel, the balance being aluminum, and the jacket consists of a zirconium alloy containing from 1.2 to 1.6% by weight of tin, from 1.08 to 1.17% by weight of iron, from 0.06 to 0.14% by weight of chromium and from 0 to 0.07% by weight of nickel.

7. A fuel element for neutronic reactors having a high corrosion resistance to water of elevated temperature and superatmospheric pressure, consisting of a core and a jacket around said core, said core consisting of an alloy composed of from 1 to 2% by weight of plutonium, from 1.0 to 1.4% by weight of nickel, from 0.9 to 1.1% by weight of silicon, from 0 to 0.7% by weight of iron, and aluminum and said jacket consisting of a corrosion-resistant zirconium alloy.

8. The fuel element of claim 7 wherein the core consists of 1.8% by weight of plutonium, 1.2% by weight of nickel and 1.0% by weight of silicon, the balance being aluminum, and the jacket consists of a zirconium alloy containing from 1.2 to 1.6% by Weight of tin, from 1.08 to 1.17% by weight of iron, from 0.06 to 0.14% by weight of chromium and from 0 to 0.07% by weight of nickel.

9. A corrosion-resistant alloy consisting of from 1 to 10% by weight of plutonium, from 2 to 3.5% by weight of nickel, up to 0.7% by weight of iron, the balance being aluminum.

10. A corrosion-resistant alloy consisting of from 1 to 10% by weight of plutonium, from 1 to 1.4% by weight of nickel, from 0.9 to 1.1% by weight of silicon, up to 0.7% by weight of iron, the balance being aluminum.

References Cited in the file of this patent UNITED STATES PATENTS Thomas et al. Dec. 4, 1956 Cofl'lnberry Aug. 25, 1959 OTHER REFERENCES 

1. A CORROSION-RESISTANT TERNARY ALLOY CONSISTING OF FROM 1 TO 10% BY WEIGHT OF POLUTONIUM, NICKEL, AND FROM 2 TO 3.5% BY WEIGHT OF ALUMINUM. 